The present invention relates to a multiplex transmission method in which a multiplex transmission apparatus switches its operation mode from a power saving operation mode to a normal operation mode to perform multiplex data transmission.
Conventionally, a plurality of multiplex transmission apparatuses (hereinafter referred to as xe2x80x9cnodesxe2x80x9d) are connected to a transmission line consisting of, for example, twisted pair wires to construct a multiplex transmission network system. Multiplex transmission with such a system is adopted, for example, in a local area network (LAN) in an automobile provided for performing multiplex data transmission using CSMA/CD transmission method.
Each node has a microcomputer including a CPU core, a transmission control circuit, and an input port. The microcomputer is connected to a generating circuit and operates with a clock signal generated by the generating circuit.
A signal coming in from a switch or the like is forwarded to the input port of the microprocessor through an input circuit. When a value at the input port has changed or predetermined time has passed, the CPU core instructs the transmission control circuit to transmit a frame. Receiving from the CPU core an instruction to transmit a frame, the transmission control circuit arranges data forwarded from the CPU core in the form of a frame and transmits the frame to other nodes connected to the transmission line.
As an operation mode, each node has a normal operation mode in which it does normal operation for performing data transmission with other nodes and a power saving operation mode in which it operates with saved power to thereby reduce power consumption when the system does not need to be active. Specifically, the power saving operation mode is an operation mode in which the operation of the generating circuit is stopped to thereby reduce power consumption, for example, when a signal coming from a switch continues to be inactive and the node itself continues to be inactive.
When the node is in the power saving operation mode, if a signal coming from a switch or the like changes or a frame for mode switch instruction comes in from the transmission line (for example, bus), the node needs to become active. Therefore, the CPU core makes the generating circuit start operation again to thereby switch the operation mode to the normal operation mode (This mode switching is referred to as xe2x80x9cwaking-upxe2x80x9d). For this purpose, the node is adapted to take in, with its input port for waking-up, a frame forwarded through the transmission line and a signal coming from a predetermined switch, and when the nodes detects a change of the signal or a frame""s coming in while the node is in the power saving operation mode, the node wakes up.
After waking up, the node immediately transmits a frame for wake-up instruction to the other nodes to thereby inform them that the node itself has switched to the normal operation mode, and then performs normal data transmission.
According to the above transmission method, however, if the whole system is in the power saving operation mode, and then if one node transmits a frame for waking-up to the other nodes, each of the other nodes wakes up and then transmits a frame for waking-up. Thus, according to the above transmission method, traffic on the transmission line may much increase at the time of waking-up.
As a result, at the time of waking-up, a trouble may happen such that a node which seeks to transmit data about a change of a switch is made to wait, and it takes time to perform data transmission.
For example, if the above transmission method is used in a multiplex data transmission system provided in an automobile, the following trouble may happen: when a door is to be opened with a door lock switch, a node which seeks to transmit data about a change of the door lock switch is made to wait due to an increase in traffic at the time of waking-up. Thus, there may be a time lag of 1 to 2 seconds after the door lock switch is handled until the door lock motor starts to operate. If the multiplex transmission system includes a large number of nodes, the time lag may be so large as 3 to 5 seconds. Thus, there are difficulties in using the above transmission method in a multiplex transmission system for an automobile, because in an automobile, for example, an actuator (lump, motor, or the like) needs to start to operate immediately when a switch is handled.
The present invention has been made in consideration of the above problems. An object of the present invention is to provide a multiplex transmission method which can reduce an increase in traffic at the time of waking up and perform data transmission rapidly.
Another object of the present invention is to reduce unnecessary transmission of a wake-up instruction.
Another object of the present invention is to transmit the latest predetermined data at the time of transmitting a wake-up instruction.
In order to achieve the above objects, according to the present invention, a plurality of nodes each having, as an operation mode, a power saving operation mode and a normal operation mode are connected to a common transmission line, and each of the nodes switches, based on a wake-up instruction, from the power saving operation mode to the normal operation mode to perform multiplex transmission. Each node judges whether a wake-up instruction has come from itself or from another node, and only when the wake-up instruction has comes from itself, transmits a wake-up instruction at regular intervals.
Specifically, when a node wakes up due to change of a signal coming into the node, the node transmits a wake-up instruction, but when the node wakes up due to a frame which has come to the node from another node, the node does not transmit a wake-up instruction in order to reduce an increase in traffic at the time of waking up and soon perform normal data transmission.
It is desirable that each of nodes which have received a wake-up instruction transmitted by one node transmits an acknowledgment signal, and the one node which is transmitting the wake-up instruction stops transmitting the wake-up instruction when it has received acknowledgment signals from all the other nodes in order to avoid unnecessary transmission of the wake-up instruction.
It is desirable that a node transmits predetermined data along with a wake-up instruction at the time of transmitting the wake-up instruction, and that the data transmitted along with the wake-up instruction is data renewed to be the latest so that the node may always transmit the latest data.